Upon the widespread use of recent portable terminals, e.g., cellular phones, a size and weight reduction has become a key factor for their development. A high output power amplifier has been recognized as a key part.
As a heterojunction bipolar transistor (Heterojunction bipolar transistor, hereinafter called “HBT”) has a high current gain β, a GaAs-system HBT having an emitter comprised of AlGaAs and a base comprised of GaAs is often used in a high output power amplifier for a cellular phone along with its high-speed property.
In order to realize an increase in output, the HBT needs a large emitter size for the purpose of obtaining a predetermined output. In order to obtain such a large emitter size, the so-called multifinger configuration is needed wherein a plurality of HBTs made up of emitters narrow in width to reduce base resistances are connected in parallel. An HBT having such a multifinger configuration will hereinafter be called a “multifinger HBT”, and individual HBTs that constitute this multifinger HBT, will hereinafter be called “basic HBTs.”
FIG. 11 is a block diagram showing a configuration of a conventional high output power amplifier.
In FIG. 11, reference numeral 100 indicates a power amplifier, 102 indicates a multifinger HBT which serves as a first stage of the power amplifier 100, and 104 indicates basic HBTs of the multifinger HBT 102. X11, X12, . . . , X1m are connected in parallel as the basic HBTs. Each of the basic HBTs 104 comprises an HBT 104a and an emitter resistor 104b connected in series with an emitter electrode of the HBT 104a. Signal power is inputted to a base electrode of the HBT 104a in each basic HBT 104, and an amplified signal output is outputted from a collector electrode of the HBT 104a. The emitter electrode of each HBT 104a is grounded via the emitter resistor 104b. 
Reference numeral 106 indicates a multifinger HBT which serves as an output stage of the amplifier 100. Reference numerals 108 indicate basic HBTs of the multifinger HBT 106. X21, X22, . . . , X2n are connected in parallel as the basic HBTs. Each basic HBT 108 is identical in configuration to each basic HBT 104 of the multifinger HBT 102. The basic HBT 108 comprises an HBT 108a and an emitter resistor 108b connected in series with an emitter electrode of the HBT 108a. Signal power is inputted to a base electrode of the HBT 108a in each basic HBT 108, and an amplified signal output is outputted from a collector electrode of the HBT 108a. The emitter electrode of the HBT 108a is grounded via the emitter resistor 108b. 
Reference numeral 110 indicates an input terminal, 112 indicates an output terminal, 114 indicates a source voltage terminal, 116 indicates a base bias circuit, and 118, 120, 122, 124, 126, 128 and 130 indicate matching circuits, respectively.
When the HBT 104a and the HBT 108a each are brought to a high temperature, forward voltages VBE of their bases become low. A compound semiconductor substrate such as GaAs or the like is high in thermal resistance. Therefore, when a current concentrates on one HBT 104a and one HBT 108a due to any cause where a large number of HBTs 104a and HBTs 108a are connected and disposed in parallel, a problem arises that heat generated in each of the HBT 104a and HBT 108a will rise and VBE will decrease, and the current will concentrate further more on such a portion.
Now, the thermal resistance ΘTH is defined by the following expression: ΘTH=ΔTj/ΔP. In the expression, Tj indicates a junction temperature, and P indicates power.
It has been reported by, for example, IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 43, NO. 2, FEBRUARY 1996, pp220-227 that since the basic HBTs are thermally ununiform therebetween, the concentration of a current on a certain one basic HBT occurs and consequently a sudden change in operating current takes place.
When the current concentrates on specific HBTs104a and 108a in the multifinger HBTs 102 and 106, the number of operated HBTs 104 and 108 is reduced and hence power proportional to a size cannot be obtained. Therefore, resistors are inserted in series with their corresponding bases or emitters of the HBTs 104a and 108a to prevent the concentration of the current on the specific HBTs 104a and 108a, thereby reducing a current amplification gain so as to restrain the concentration of the current.
In the power amplifier 100, the emitter resistors are inserted in series with the emitter electrodes of the HBTs104a and 108a to constitute the basic HBTs104 and 108.
However, when the resistors are inserted in series with the bases or emitters of the HBTs 104a and 108a, the power amplifier 100 is degraded in performance.
That is to say, since the mere insertion of the base resistors will result in the insertion of the resistors into the inputs, losses are produced and noise characteristics are degraded due to their losses. Further, the gain of each HBT is reduced due to the occurrence of the losses.
On the other hand, when the emitter resistors are simply inserted as in the basic HBTs 104 and 108, Vce is reduced due to voltage drops developed by the emitter resistors 104b and 108b, thereby causing degradation in power efficiency.
Thus, the insertion of the resistors in series with the bases or emitters to reduce the current amplification gain so as to restrain the concentration of the current is effective in preventing the thermal ununiformity of each basic HBT. However, a problem arises in that when the resistors are simply inserted in series with the bases or emitters, noise characteristics are degraded, the gain of each HBT is lowered, and power efficiency is reduced.
Further, a configuration of an HBT device using an emitter ballast resistor and a configuration of an HBT device using a base ballast resistor have been described in Japanese Patent Laid-open No. Hei 8(1996)-279561 as the known reference.
Furthermore, a configuration of an HBT device using an emitter ballast resistor has been described in IEEE MTT-S Digest WE2A-6, 1994, p687-p690.
FIG. 12 is a block diagram of a conventional HBT multistage amplifier configured using only HBT devices which make use of emitter ballast resistors.
In FIG. 12, a multifinger HBT102, which serves as a first stage of an HBT multistage amplifier 140, and a multifinger HBT106, which serves as an output stage, are made up of basic HBTs having emitter ballast resistors connected in series with emitter electrodes in a manner similar to FIG. 11. All of intermediate stages provided between the first stage and the output stage are also made up of multifinger HBTs comprised of basic HBTs having emitter ballast resistors connected in series with emitter electrodes.
FIG. 13 is a block diagram of a conventional HBT multistage amplifier constructed using only HBT devices using base ballast resistors.
In FIG. 13, an HBT multistage amplifier 142, i.e., all of a first stage to an output stage including intermediate stages are made up of multifinger HBTs 144 comprised of basic HBTs having base ballast resistors.
Thus, the conventional HBT multistage amplifiers 140 and 142 are made up of only the multifinger HBTs using the emitter ballast resistors, and only the multifinger HBTs using the base ballast resistors, respectively.
Since resistors each are inserted between an emitter and a ground, a multifinger HBT using emitter ballast resistors generally causes an increase in loss on both the input and output sides of an amplifier as compared with the case in which no resistors are provided. Since the loss is inflicted even on the output side, the amplifier is reduced in output power and efficiency characteristic.
Since resistors each are inserted between a base and a bias terminal, a multifinger HBT using base ballast resistors causes no loss on the output side. Therefore, the output power and efficiency characteristics are satisfactory as compared with the multifinger HBT using the emitter ballast resistors. Since, however, a large loss is inflicted on the input side, noise characteristics are degraded as compared with the multifinger HBT using the emitter ballast resistors.
In summary, the multifinger HBT144 using the base ballast resistors is better in output power and efficiency characteristic more than the multifinger HBTs 102 and 106 using the emitter ballast resistors; the multifinger HBTs 102 and 106 are better in noise characteristic than the multifinger HBT144.
Therefore, a problem arises in that the HBT multistage amplifier 140 comprising only the multifinger HBTs using the emitter ballast resistors is good in noise characteristic but becomes poor in output power and efficiency characteristic. On the other hand, a problem arises in that the HBT multistage amplifier 142 made up of only the multifinger HBTs144 using the base ballast resistors is good in output power and efficiency characteristic but becomes poor in noise characteristic.
An amplifier used in a transmitter includes specs of receiving-band noise as well as high-output and high efficiency characteristics and requires even low noise characteristics. It is therefore necessary to simultaneously realize the high-output and high-efficiency characteristics and the low noise characteristics. A problem arises in that the HBT multistage amplifier 140 or 142 having the conventional configuration is not capable of simultaneously realizing the high-output and high-efficiency characteristics and the low noise characteristics but realizing only either one of them.
The present invention has been made to solve such problems. A first object of the present invention is to provide a high-frequency semiconductor device provided with an amplifier circuit which lessens degradation in high frequency characteristic and provides high thermal stability by configuring an amplifier circuit which lessens degradation in noise characteristic and a reduction in HBT's gain and provides less reduction in power efficiency while the concentration of a current on a multifinger HBT is being restrained.
A second object of the present invention is to provide a high-frequency semiconductor device equipped with an amplifier circuit which is high in stability relative to the concentration of a current and high in reliability.
A third object of the present invention is to provide a high-frequency semiconductor device provided with an HBT multistage amplifier capable of simultaneously implementing high-output and high-efficiency characteristics and low noise characteristics.
Incidentally, while Japanese Patent Laid-open No. Hei 10(1998)-98336 has described an invention wherein a bias circuit capable of setting an operating current of an output transistor of an amplifier circuit regardless of a source voltage and in proportion to only a base-to-emitter voltage is used in a high-frequency amplifier circuit, whereby variations in saturated output level and distortion characteristic are reduced even if operating environments such as a source voltage, ambient temperatures vary, the inventions to be mentioned below are not described.